Access networks connect business and residential subscribers to the central offices of service providers, which in turn are connected to metropolitan area networks (MANs) or wide area networks (WANs). Access networks are commonly referred to as the last mile or first mile, where the latter term emphasizes their importance to subscribers. In today's access networks, telephone companies deploy digital subscriber loop (xDSL) technologies and cable companies deploy cable modems. Typically, these access networks are hybrid fiber coax (HFC) systems with an optical fiber based feeder network between central office and remote node and an electrical distribution network between the remote node and subscribers.
These access technologies are unable to provide enough bandwidth to current high-speed Gigabit Ethernet local area networks (LANs) and evolving services, e.g., distributed gaming or video on demand. Future first-mile solutions not only have to provide more bandwidth but also have to meet the cost-sensitivity constraints of access networks arising from the small number of cost sharing subscribers.
In so-called FTTx access networks, the copper based distribution part of access networks is replaced with optical fiber, e.g., fiber-to-the-curb (FTTC) or fiber-to-the-home (FTTH). In doing so, the capacity of access networks is sufficiently increased to enable the provision of broadband services to subscribers. Due to the cost sensitivity of access networks, these all-optical FTTx systems are typically unpowered and consist of passive optical components, e.g., splitters and couplers. Accordingly, they are called passive optical networks (PONs). PONs attracted a great deal of attention well before the Internet spurred bandwidth growth. There are generally two types of PONs that are defined by international standard bodies. Ethernet Passive Optical Networks, known as EPON, is ratified by the IEEE 802 standards committee. Gigabit Passive Optical Networks, known as GPON is defined by ITU-T.
An example of a topology for a PON is illustrated in FIG. 1. Typically, a PON has a physical tree topology with the central office located at the root and the subscribers connected to the leaf nodes of the tree. In FIG. 1, at the root of the tree is an optical line termination (OLT) 10 which is the service provider equipment residing at the central office. The OLT 10 is the gateway that connects to the global Internet 34. The PON connects the OLT 10 to multiple optical line units (ONUs) 22,24,26 through a 1:N optical splitter/combiner 14. In the illustrated example, there are N ONUs, but only three are illustrated, specifically labeled “ONU 1” 22, “ONU 2” 24, . . . , “ONU N” 26. More specifically, a shared optical fiber 12 connects the OLT 10 to the 1:N optical splitter/combiner 14, and a respective optical fiber 16,18,20 connects the 1:N optical splitter/combiner to each ONU 22,24,26.
An ONU can serve a single residential or business subscriber, referred to as Fiber-to-the-Home/Business (FTTH/B), or multiple subscribers, referred to as Fiber-to-the-curb (FTTC). Each ONU can be connected to one or more multimedia devices such as telephone, computer and television set. In a specific example ONU 22 is shown connected to telephone 28, computer 30, television 32.
Due to the directional properties of the optical splitter/combiner 14, the OLT 10 is able to broadcast data to all ONUs 22,24,26 in the downstream direction. In the upstream direction, however, ONUs 22,24,26 cannot communicate directly with one another. Instead, each ONU 22,24,26 is able to send data only to the OLT. Thus, in the downstream direction a PON may be viewed as a point-to-multipoint network and in the upstream direction, a PON may be viewed as a multipoint-to-point network.
It is important to note that the upstream bandwidth is time shared by all ONUs, and only one ONU can transmit data to the OLT at a time to avoid traffic collision. The OLT arbitrates which ONU can transmit data at a time and the duration of such transmission. This operation is known as dynamic bandwidth allocation or DBA.
Since multiple ONUs share the same fiber bandwidth, it is important to protect any fiber failure since such an event would disrupt the service to many customers. Particularly, it is important for mission critical applications. Loss of customer traffic for a certain period of time could lead to significant revenue loss for service providers which offer services over such a network.
Protection switching can be used to protect against fiber failure by providing a redundant path. An example of this is illustrated in FIG. 2. The PON of FIG. 2 is basically the same as the PON of FIG. 1, and common reference numbers have been used where appropriate, with the following exceptions:
a) instead of a single fiber 12 as was the case in FIG. 1, there is a trunk fiber pair 40 between the OLT 10 and a remote optical splitter 42, where one fiber is configured to be active and the other standby; and
b) the remote optical splitter 42 has a 2:N splitting/combining function as opposed to the 1:N splitting/combining function of FIG. 1.
This configuration is known as 1:1 protection switching configuration, in which the active fiber initially carries data traffic. Once the active fiber cut has been detected, data traffic will be restored onto the standby fiber.
Protection switching generally has two phases. The first phase is to detect a fiber cut, and the time to detect such an event is referred to as the fiber cut detection time. Following the first phase, the second phase is to restore the impacted traffic onto the standby fiber, and the time to restore the traffic completely is referred to as the traffic restoration time. The sum of the detection time and restoration time, during which data traffic is lost, is collectively referred to as the protection switching time. Protection switching time is the most important measure for a protection switching scheme. The established industry standard calls for it to be less than or equal to 50 ms. This could be relaxed to sub-100 ms for Internet real time applications such as voice over IP.
For downstream traffic, the OLT broadcasts content for all of the ONUs on a single broadcast channel. Addressing is used to identify the appropriate ONU for each packet. For upstream traffic, the OLT controls the time sharing of the bandwidth between the ONUs by indicating to each ONU when it is allowed to transmit upstream traffic thereby avoiding collisions. When a new ONU enters into the PON network the OLT will not be aware of it and will not allocate any upstream capacity to it. To allow for new ONUs to join, the OLT periodically allocates a “discovery window”. The allocation is signaled on the downlink broadcast channel, so all ONUs including a new ONU that has not yet registered can receive it. A discovery window is a time that is specifically reserved for new ONUs to join. The new ONU first needs to register with the OLT. This is achieved by the ONU sending a registration request to the OLT during one of the discovery windows. Typically, a three-way handshaking is performed to complete the registration process. During this process, the ONU is assigned a unique ID by the OLT. This ID or related information is carried in the data and control traffic to identify which ONU is sending (upstream) and to identify which ONU is an intended receiver (downstream).
Once the ONU is registered, applications need to establish certain control and management protocols between the OLT and the registered ONU, before normal traffic can be sent or received by the ONU. Authentication protocol (e.g. 802.1x), keep-alive protocol (e.g. OAM protocol), and multicast registration protocol (e.g. IGMP snooping) are good examples of such protocols.
Each ONU subscribes to services offered by the service provider, based on a so-called service level agreement (SLA). Among other policy terms and conditions, the SLA contains the quality of service (QoS) requirements for bandwidth and delay, which is referred to as a traffic profile. The bandwidth information includes committed information rate (CIR), peak information rate (PIR), and burst size (BS). The delay information includes bounded delay and jitter. The traffic profile is used for the purpose of bandwidth allocation among ONUs in the upstream direction of the PON system. The objective of the bandwidth allocation is to ensure that ONUs will gain fair access to the available bandwidth based on their respective traffic profiles. Based on the traffic profiles, the OLT will periodically send bandwidth allocations to the registered ONUs, in response to bandwidth requests.
The OLT periodically allocates a discovery window that allows ONUs to register. When more than one ONU attempts to register simultaneously, a collision will occur. Each ONU then backs off randomly to avoid a possibility of future collisions.
In the 1:1 protection configuration as illustrated in FIG. 2, there is a short period of time that an ONU experiences the loss of light when the active fiber is cut. It is extremely difficult, if not prohibitive, to preserve the timing information across the switchover at the ONU. In other words, ONU re-registration in the event of fiber cut is inevitable. Once the ONU detects a fiber cut event, it will deregister with the OLT and start a new registration process from the beginning. Such a process is time consuming and could take a few seconds or a minute to complete. The most time consuming functions to perform are related to the establishment of control and management protocols between applications running on the ONU and the OLT. The typical three-way handshaking registration itself can be achieved very fast, given the high speed link between the OLT and an ONU.
Similarly, when the OLT detects the loss of communication with one or more ONUs, it will clear the state information associated with them such as ONU ID, registration information and use profiles. This will lead to the reset of associated applications in the control and management plane. Typically, the OLT detects the communication loss through one of several keep-alive protocols such as MPCP or OAM in the case of EPON, and the detection time is in the neighbourhood of 50 milliseconds (MPCP) or one second (OAM) depending on the protocol used. The normal detection mechanism is certainly inefficient from protection switching perspective.